Intrinsic X-ray column density distribution as a function of redshift for both Galactic models. The panel on the left shows the N_H(z) distribution obtained with the LAB survey. We note that the 41 GRBs of the extended sample, shown in a lighter colour with respect to the (blue) dots of the BAT6 sample, do not affect the increasing trend that came out in C12. The panel on the right reports the distribution, computed with the new W13 Galactic absorption model. The triangles, representing N_H(z) for the 18 GRBs with no redshift measure, should be considered as lower limits, which increase with redshift as ~(1 + z)^2.4. The error bars were calculated within XSPEC at 90% confidence level (Δχ² = 2.71), as were the upper limits.